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Gas Turbine Compressor Stall

  1. Apr 16, 2014 #1
    Hi,
    I went through many resources but I am not able to understand as to what stall/surge means for an axial compressor means. I am not able to imagine or correlate it. I want to physically understand what happens in a compressor when it is stalled. For eg: Can a ceiling fan stall when the angle of its blades it too large? If yes then what would happen in practice (things which we can observe when the fan stalls??)
    An observable analogy is what I want..

    Also why is stall so detrimental..this in an interlinked question (related to understanding of stall) but still I would go ahead and ask it.

    In addition are there any good resources which would provide a clear understanding of things like compressor map, performance etc...

    Thanks...
     
  2. jcsd
  3. Apr 17, 2014 #2

    Baluncore

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    The individual blades are airfoils, so I think you should focus attention on the angle of attack and the stall of an airfoil. While fluid flow is laminar the airfoil will have lift but when the fluid breaks away from the surface the airfoil will stall. If there is insufficient angle of attack, the airfoil will not transfer energy to the fluid. When the airfoil has an extreme negative angle of attack, the lift will be reversed and will tend to block the flow of fluid.

    It is possible for the flow through a multi-stage compressor to become unstable and oscillate between these extreme situations.
    Standing or travelling waves of high and low pressure or flow can propagate along the stages.
     
    Last edited: Apr 17, 2014
  4. Apr 17, 2014 #3
  5. Apr 21, 2014 #4
    Hi Baluncore and Spinnor..thanks for your replies..but i am still not able to clearly get what physically happens when one says the compressor is stalling/stalled?
     
  6. Apr 22, 2014 #5

    Baluncore

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  7. Apr 22, 2014 #6
    Hi..Yes I understand what is angle of attack and stall...But what exactly happens when a boundary layer separation occurs? how the fluid moves then? how the blades and vanes react at this particular moment when boundary layer is detached?
    My question is there any analogy or something which could help me visualize/imagine this phenomenon.
     
  8. Apr 22, 2014 #7
    The video below is very basic but may make something click. The one wiki article mentioned that when this happens the solution is to reduce fuel flow to the combustion chamber. I would ask myself what happens as fuel flow to the engine is raised above normal maximum flow rates?



    Also see,

    https://www.youtube.com/results?search_query=Jet+engine+compressor+stall
     
    Last edited by a moderator: Sep 25, 2014
  9. Apr 22, 2014 #8

    Baluncore

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    Before you get into the complexities of turbine stall, I think you should focus on understanding the airfoil stall situation.
     
  10. Apr 24, 2014 #9
    As you lose laminar flow across the compressor blades that stage can no longer keep up enough pressure differential to keep the air flow along the engine, as that happens the air on the HP side of the stage reverses it's flow. A poor analogy but think what happens if you put your finger over the end of a bicycle pump, as you move the piston down you need to increase the pressure on the handle, if you release the handle it springs back.
     
  11. Apr 25, 2014 #10
    Thanks Jobrag..
    It was really helpful...
     
  12. May 13, 2014 #11
    Nope..still confused...How does the air move when the compressor stalls? It separates from the individual or a group of airfoils...but what does it do after that? How does it move in the compressor annulus?
    What happens to the airflow after the compressor stalls? Does it still go in, moving at same speed or it slows down....or it remains there as such in the compressor, stationary (if we block the air from the combustion chamber)?
    How does the mass flow rate gets affected? What about the pressure ratio..how does it vary?
     
    Last edited: May 13, 2014
  13. May 13, 2014 #12
    Two or three things happen,
    First, there is huge surge in vibration, so the engine trips (stops) on high vibration.
    Second, because there's a massive reduction in airflow through the hot end EGT spikes enormously and the engine trips on high temperature trip.
    Third if your unlucky the reverse flow on the turbine blades rips them out of their mountings and the engine is wrecked.
     
  14. May 13, 2014 #13

    AlephZero

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    An axial flow compressor with a high compression ratio is potentially an unstable air flow through a pipe. The only thing that keeps the flow stable is that you continuously force air into the inlet, and its only option is to escape from the smaller outlet at higher presssure.

    If anything messes up the aerodynamics that are forcing the air into the compressor, the high pressure air inside the pipe does what it really wants to, which is escape from both ends as fast as possible. The result is a bang, usually flames coming out of both ends of the engine at once, and internal mechanical damage that can range from negligible to catastrophic.

    If the damage is negligible, the stable flow pattern may recover, or it may try to recover and produce repeated surges and bangs at a frequency of around one per second until something breaks. If the engine was running at high power, suddenly reducing the air flow while still pumping the same amount of fuel into the combustor is also not good news....
     
  15. May 13, 2014 #14
    I understand it better now....the simple analogy of pipe flow is really helpful..
     
  16. May 14, 2014 #15
    Hi...i thought of an analogy(for the whole compressor as a whole)....not sure if its correct..
    A stalled compressor is like an automobile pushing hard against a fixed wall....the wall wont allow the vehicle to move...but still if we continue to push, the engine heats up badly and does no useful work...
    now in place of the engine we've the compressor which is trying hard to rotate the turbine...but since it is stalled, it would provide no useful work and just get heated up...
    Does it sound ok, guys?
     
  17. May 14, 2014 #16

    AlephZero

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    That's back to front. The turbine is driving the compressor.
     
  18. May 15, 2014 #17
    Got that wrong...so the effect of stall (in above analogy) would be that the turbine is driving hard the compressor and in the process gets heated up.
     
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